No 18, hell no 16 year old from the modern world not to consider some distant alien world of super science is going to call herself "girl" unless she's trying to be sarcastic like the Legion does, hells, for any female to willing refer to themselves as a girl rather than a woman they'd have to be 12 or younger.
Hayden would be in a movie called Supergnome before she's in a movie called Supergirl. There's nothing wrong with short women but the physicality of Supergirl is an archetype for sure.
Dakota can't be Supergirl because she's already been Wonder Woman. If that doesn't discredit her, then how about this... Once apon a time she was (young) Ally Mcbeal, and it's near impossible to wash that sort of bitchiness off ones soul.
How old is Elle Fanning these days?